The Newport Gwent Dragons full-back will move from Rodney Parade at the end of this season, the Ospreys announced.
Evans, 25, won two caps on Wales' tour to North America in 2009.
"This is a great opportunity for me," he said on the club's official website.
Rugby
Ospreys snatch win with late Rhys Webb double
30/04/2016 AT 15:46
"The Ospreys have been the most successful team in Wales over the last 10 years, winning leagues and producing players for the national team.
"There is some real world-class talent in the squad so it's a big challenge for me personally, but I can't wait to get involved and fight for my place."
Rugby
Ospreys pile on the points to stay in European Champions Cup hunt
15/04/2016 AT 21:02
Rugby
Ospreys pile on the points to stay in European Champions Cup hunt
15/04/2016 AT 21:01